Except for the issue of whether the lot is already subdivided (or needs further municipal approvals to do so) and whether your parents will convey it to you before you decide to build a house there and then rent or sell the lot, this isn't really a legal question, but an economic one.
How much will it cost to build a house there? What is the market and going rate for buildable lots, home rentals or sales of existing homes in the area where you're thinking of building? Talk to some realtors to get an idea of whether what you're thinking of doing is a good investment and whether it's better to build, rent or sell.
Don't do anything unless and until your parents decide whether to convey you the lot and deed it to you. You will want a lawyer to help you with that part of the process, and there are often tax and estate implications benefits when property is transferred between family members rather than between sellers and buyers who aren't related.
The decision to build the house and rent or sell it is mostly an economic decision. Having said that there are legal issues that you need to review with Texas licensed attorney.
The first issue is the ownership of the lot. Are you buying the lot from your parents? Are you jointly developing the lot with them? Are they gifting the lot to you? This can have significant risks and tax implications for you and your parents. Before proceeding you need to review the options with an attorney.
The second legal issues would be zoning, that is are the lots of legal size for houses. Have the lots been properly subdivided. By zoning your parents may have two lots just because of the size of the house. These issues can take several months depending on the municipality. An attorney or architect can help you review the lots.
Lastly you need to review if you can self perform the construction without a license in TX and your local authority in this application.
All good questions--none that can well be answered here. Depends on many factors not posted. See a San Antonia area property attorney before taking any steps or investing any money.
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